Pickup trucks may include a cargo box that may have three sides defined by the structure of the vehicle and a fourth side at the back of the vehicle that is typically provided as tailgate. The tailgate is usually hinged at its lower edge so that it opens by pivoting out and down to a horizontal orientation. When dosed, the tailgate, the box bed, and other three sides of the box define a cargo area with a fixed volume. When the tailgate is open, it presents a horizontal surface that extends off the back of the bed, but leaves the back of the box volume open, and so is generally not useable for carrying unsecured cargo.